A known steam turbine rotor can be exposed to a variety of conditions along its axial length. As no one material is optimum for the differing conditions, it is desirable to assemble the rotor from sections each made of a different material. In order to form a complete rotor from the sections, the sections can be joined.
One method of joining rotor sections is by welding. However, some materials are difficult or impossible to weld. In such cases either different materials can be selected or alternate joining methods provided. One alternative joining method involves the use of bolts. European patent EP 1378629 A1 discloses a method involving individually bolting together forged sections of a high-temperature steam turbine. As the bolts are exposed to the full stress of the join, a significant number of bolts are required to maintain join integrity.
Another method is disclosed in DE 343462 C. The method involves joining rotor sections by screwing a threaded boss, which extends from one section, into a tapped cavity of an adjacent section. In order to prevent the joined sections from separating axially on account of varying bending stresses and vibration, a locking member is passed transversely through both the boss and adjacent disk so as to extend across the completed diameter of the rotor. In one arrangement, two members are provided. These members are arranged 90° from each other and perpendicular to the rotor axis.
When this method is applied to large steam turbines, due to rotor size and the length specification of the locking members, pre-drilling of cavities for the locking members through the rotors can be carried out during manufacture of the rotor sections. As it is difficult to predetermine the optimum location for the locking cavities before making the join, there may be undertorquing of the join. As a result, the locking members should be designed to withstand some of the specified joining force, increasing their size and further complicating the design.